A high definition television (HDTV) system is generally understood to be a system for processing a television signal having approximately double the horizontal and vertical image resolution of a standard television signal, e.g., NTSC, or greater resolution. An HDTV signal also may exhibit a greater image aspect ratio, e.g., 16.times.9, compared to the 4.times.3 aspect ratio of a standard NTSC television image.
In a simulcast television broadcast system, two versions of the same program material are broadcast simultaneously via separate standard 6 MHz channels. One of the two program versions contains standard definition NTSC information broadcast on one channel, while the other contains high definition information broadcast on the other 6 MHz channel. In practice, a simulcast system may utilize two adjacent 6 MHz NTSC channels, e.g., VEF channels 3 and 4, to convey the standard and HDTV information respectively. The high definition version of a simulcast system can be implemented in a single 6 MHz channel by the use of signal encoding and time compression techniques. The standard NTSC information and the HDTV information are received independently by respective standard NTSC and HDTV receivers. When standard NTSC receivers are eventually replaced by HDTV or dual-standard receivers, perhaps in 15-20 years, the channels used by standard NTSC television signals will become available for other purposes. Thus the simulcast concept prevents the vast number of preexisting standard NTSC receivers from becoming obsolete as soon as HDTV broadcasting is introduced, and permits expanded broadcasting services in the future when the channels occupied by standard NTSC signals become available.
A simulcast system differs from a so-called augmentation system in that an augmentation system requires the continuing use of two channels. One channel conveys standard NTSC information, while the other channel contains predetermined augmentation information which, when combined at an HDTV receiver with the standard NTSC information from the first channel, produces a high definition television signal.
It is important to significantly reduce or eliminate co-channel interference between the standard and HDTV signals transmitted from different locations on the same channel. A system according to the present invention addresses this matter.